The invention is particularly adapted for electric heating elements for heating liquids in a container, such as the commercially known water heater elements adapted to heat water in a tank. The heater is held to the wall of the tank by a mounting flange which is rigidly connected to the heater sheath.
The heater assembly comprises a conventional sheathed electric heating element, the components of which include a tubular metal sheath; a resistance member, such as a helical resistance wire, within the sheath and electrically insulated from the inner wall surface thereof by compacted refractory material; and metal terminal pins electrically connected to the ends of the resistance wire. A mounting flange is rigidly connected to the end (or ends) of the sheath by a mechanical connection, such as by staking. Representative of the prior art of connecting a metal flange to a sheathed heating element by a staking operation is U.S. Pat. No. 2,670,529.
An end seal of some type is usually disposed within the open end of the sheath, and U.S. Pat. Nos. 2,489,998 and 2,861,162 disclose rubber bushings held within the sheath end. In mechanical staking of a mounting flange or screw plug to a sheathed heater, the staking displaces the metal of the flange or plug inwardly towards the tubular leg of the heater. However, in order to effect a satisfactory seal between the engaging surfaces of the mounting flange and tubular leg, the latter must have sufficient internal backup to prevent it from collapsing during the staking operation.
It has been found that a relatively soft rubber bushing within the end of the tubular leg does not provide a satisfactory backup and for that reason relatively hard bushings, such as hard plastic or rubber were used. The hard bushings provided satisfactory backup but did not always provide an effective seal. Since finished tank assemblies are presently being tested with air pressure to check for leaks, it is necessary to provide an effective seal that will not leak air at approximately 100 p.s.i.
Our invention enables the use of the softer rubber bushings to provide the effective air seal, and the invention makes use of the fact that rubber behaves similar to fluids when subjected to pressure. By applying end pressure axially on the rubber bushing within the end of the tubular element, it is possible to transmit that pressure to the side wall of the tube to provide the necessary backup.